The performance of ultrafast, optical time division multiplexed (TDM) networks depends strongly on the recovery of timing information. Conventional electronic clock recovery schemes often do not have sufficient speed to accommodate ultra-fast TDM networks with through-puts in the regions of hundreds of gigabits per second. One technique for ultra-fast timing employs the transmission of a clock signal simultaneously with the data and requires the receiver to separate the clock and data signals. The clock and data can be distinguished by wavelength polarization or amplitude, but each method has a problem which needs to be addressed.
If the clock and data have different wavelengths, then fiber dispersion causes clock skew which varies with transmission distance. If the clock and data have different polarizations, then polarization control is required at the receiver. If the clock and data have different amplitudes (and the clock occupies a designated time slot in the TDM frame or packet), an ultra-fast intensity thresholding device is required to extract the clock pulses at the receiver.
Non-linear optical loop mirrors (NOLMs) are good candidates for optical thresholding because they exhibit ultra-fast switching capabilities. However, they often require high switching energies or long interaction lengths (up to several kilometers) to achieve the switching action. From a practical standpoint, available switching energy is generally limited because high energy pulses experience nonlinear interactions during transmission, and such interactions causing unacceptable timing jitter. For instance, picosecond duration pulses must have less than one picoJoule of energy to avoid nonlinear effects over a 10 kilometer distance.
Various designs of NOLMs can be found described in the following references: "Picosecond Soliton Propagation Using Nonlinear Optical Loop Mirrors as Intensity Filters", Smith et al., Electronics Letters, Vol 30, No. 13, pp 1084,1085, 1994; "Nonlinear Optical Loop Mirror", Doran et al., Optics Letters, Volume 13, No. 1, January 1988, pages 56-58; and "Experimental Demonstration of Optical Soliton Switching in an All-Fiber Nonlinear Sagnac Interferometer", Blow et al., Optics Letters, Volume 14, No. 14, Jul. 15, 1989, pages 754-756.
Some of the switching devices described in the aforementioned papers employ an optical loop mirror wherein a coupler that is used to input optical pulses exhibits an asymmetric coupling characteristic. The Blow et al. system employs a fiber loop in one arm of the optical mirror to achieve an unbalance in phase delays, thereby achieving an ability to output pulses on the basis of amplitude discrimination.
U.S. Pat. No. 5,493,433 to Prucnal et al., assigned to the same assignee as this application, describes an optical asymmetric demultiplexer which includes an optical loop having a nonlinear optical element positioned in one arm thereof. A coupler is positioned in the loop and injects a gating pulse which causes a change in the optical property of the nonlinear optical element. A further coupler receives a series of input optical pulses and induces in the optical loop, a pair of counter-propagating pulses in response to each input pulse. Control circuitry causes a gating pulse to be applied to the optical loop that is timed to switch the nonlinear optical element from a first to a second state after one of the pair of counter-propagating pulses has passed through the nonlinear optical element, but before the other counter-propagating pulse reaches the nonlinear optical element. Thus, one counter-propagating pulse is affected by the second state of the nonlinear optical element and the other counter-propagating pulse is not. As a result, the two pulses arrive back at the input coupler, exhibiting an offset phase. The coupler responds by coupling the out-of-phase pulse to an output fiber. If no gating signal is applied to alter the state of the nonlinear element, then the pulses which counter-propagate around the loop experience an identical phase shift, constructively interfere at the input coupler and are reflected back along the input fiber.
While the Prucnal et al. system provides extraordinarily high speed switching of optical pulses, an additional coupler is required in the optical loop mirror to achieve the switching action. Other non-linear optical loop mirrors, as indicated above, require either high switching energy or long interaction lengths.
Accordingly, it is an object of this invention to provide a nonlinear optical loop mirror which is adapted to discriminate pulses on the basis of amplitude.
It is another object of this invention to provide a nonlinear optical loop mirror which employs an intensity thresholding device that exhibits a low switching energy and a short interaction length.